Abstract

In this study, Electrospray hydrodynamics and electrical potential dependency of heat transfer coefficient were investigated by computational fluid Dynamics (CFD). VOF method was applied to solve momentum equation of these two-phase flow and Whitaker empirical relationship for gas, liquid flow on sphere was also applied to calculate the heat transfer coefficient. The results of simulation were in accordance with experiments and showed that because of domination of surface tension by gravity and electric forces, diameter of droplets and their formation time were decreased. In addition, applying electrical potential at the velocity of 0.007 m/s has led to formation of jet and small droplets of liquid. Formation time of the droplet was decreased by increasing the velocity ten times higher than the previous time, to 0.07 m/s. By using the results of hydrodynamic simulation of droplet, convective heat transfer coefficient of droplet was calculated in various electrical potentials that showed heat transfer coefficient increased by growth of electrical potential.

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Introduction

Spraying is the physical process of dispersing a liquid as a stream of droplets in a gas form. The point of spraying is maximizing the liquid impressions by increasing its surface. Sprays are the instruments to simplify the spraying process of liquids and they typically include a nozzle through which liquid gets dispersed after passing through it, depending on various factors and spray type. There are different types of sprays that include in hydraulic, gaseous, supersonic and vibrant ones (Speranza & Ghadiri, 2003). When a specific process is sensitive to droplet size and its distribution, controlling the operation conditions such as flow rate, electric field strength, viscosity and electrical conductivity is very important in order to produce the desired and uniform size of droplet. Thus, according to the ability of the electro-spray method in producing the desired and uniform droplets, replacing the conventional methods by this one will lead to an improvement in spraying process. Electro-spray method can be applied instead of conventional methods in internal combustion engines, production of biodegradation polymer particles in the micrometer and nanometer dimensions (Xie, Lim, Phua, Hua, & Wang, 2006), Preparation of immobilized enzyme gel (Watanabe, Matsuyama, & Yamamoto, 2001), spraying outturns(crops) (Speranza & Ghadiri, 2003; Watanabe, Matsuyama, & Yamamoto, 2003), color spray painting (Okuda & Kelly, 1996), and the pharmaceutical industry (Ijsebaert, Geerse, Marijnissen, & Scarlett, 1999; Jaworek & Sobczyk, 2008).